Automatic gas analysis and purging system

ABSTRACT

The contaminants in the exhaust gas emissions from a motor vehicle such as carbon monoxide (CO) and hydrocarbons (HC) are analyzed and the concentration of the contaminants together with other pertinent data is displayed in a digital manner. The exhaust gas emissions are fed into a sample cell contained within a nondispersive infrared analyzer and the absorption of an infrared radiation beam at selected wavelengths by the gas within the cell is measured. A reference cell containing a reference gas is also positioned in the infrared radiation beam path. By means of a rotating chopper disk positioned in the light path, the infrared radiation beam passes alternately through the reference cell and the sample cell and is focused at a plurality of detectors which are each sensitized to a narrow wave band by a filter and which receive the alternate sample cell and reference cell radiation pulses. Synchronization of the system is provided by a notch in the chopper disk which passes alternately between three light sources and associated photoresponsive devices. Automatic span correction is obtained by an automatic gain control circuit which maintains the reference cell output at a predetermined amplitude. The sample cell is initially filled with ambient air by a gas transport system. By measuring the sample cell and reference cell outputs with ambient air in the sample cell, and then measuring the sample cell and reference cell outputs with exhaust gas in the sample cell, a ratio can be computed which is proportional to the amount of the contaminant. Computation of the ratio in this manner results in automatic zeroing and spanning of the system. The computed ratio is then calibrated for nonlinearities in the system, and linear corrections are made for variations in ambient pressure and for variations in the temperature of the gas passing through the sample tube. Provision is also made for assuring that the vehicle being tested has achieved a predetermined engine speed and that no blockage has occurred in the gas transport system when the measurements are made. After the exhaust gas has been analyzed it is replaced in the sample cell with ambient air to prevent contamination of the system optics from the exhaust gas. A sapphire window is positioned in front of the infrared source to reduce changes in source temperature by air currents produced by the rotating chopper disk.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an automatic gas analysis system for measuringthe contaminants in the exhaust gas from the motor vehicle, andparticularly to a system using a nondispersive infrared gas analyzer inwhich the exhaust gas to be analyzed is passed through a sample cellonly during the time that measurements of the contaminant level aremade, the sample cell being purged with clean ambient air to remove theexhaust gas from the sample cell during the times when measurements arenot being taken. The present invention improves the accuracy of themeasurements by purging the exhaust gas transport system of any exhaustgas which may remain in the gas transport system, and prevents thepollutants in the exhaust gas from fouling the analyzer as would occurif exhaust gas remained in the system continuously.

2. Description of the Prior Art

Public concern with environmental pollution, particularly that caused bythe contaminants emitted in the exhaust gas emissions from motorvehicles, has resulted in the development of exhaust gas analysissystems which measure the amount of selected contaminants in the exhaustgas emissions. Many such systems use nondispersive infrared gasanalyzers in which the emissions gas from a motor vehicle is fed into asample cell and an infrared light beam passed through the sample cell,the amount of infrared light absorbed at a particular wavelength beingindicative of the amount of the selected contaminant in the exhaust gas.

It has been found that inaccuracies occur in the measurements due to thehang up of exhaust gas in the gas transmission lines and gas transportsystem including the pumps which feed the exhaust gas from the motorvehicle to the sample cell in the infrared gas analyzer. It has furtherbeen found that the pollutants in the exhaust gas cling to the walls ofthe sample cell and reduce the transmission of infrared light throughthe sample cell if the exhaust gas is allowed to remain in the samplecell for a long period of time. When the sample cell and other opticalequipment in the infrared gas analyzer become coated with pollutants,frequent cleaning of the unit is required in order to preventdeterioration in the reliability of measurements produced by the unit.

SUMMARY OF THE INVENTION

The present invention overcomes the deficiencies in prior art gasanalysis systems by providing an automatic gas analysis and purgingsystem in which ambient air is automatically pumped into the sample cellto purge the sample cell and gas transport system of exhaust gasemissions at all times except when measurements are being made of theexhaust gas contaminants in the sample cell.

In accordance with a preferred embodiment of the present invention,there is disclosed an exhaust gas analysis system in which ambient airis first fed into the gas sample cell in the nondispersive infrared gasanalyzer, and an infrared light beam is passed through the ambient airin the sample cell, the absorption of the infrared beam by the ambientair in the sample cell being measured. After the initial measurement ismade, the exhaust gas analysis system automatically removes the ambientair from the sample cell and pumps the exhaust gas from the vehiclethrough the sample cell. When a sufficient time has passed so that it isassured that the sample cell is completely filled with exhaust gas, theinfrared light beam is again passed through the sample cell andmeasurements of the contaminants in the exhaust gas within the samplecell are taken. When the latter measurements have been taken, theexhaust gas analysis system again automatically purges the sample cellwith clean ambient air to remove the exhaust gas within the sample celland gas transport system.

The automatic switching from the ambient air purge to the exhaust gasinput and back again to the ambient air purge may be under the controlof the operator of the system, or may be performed automatically by adigital or analog data analysis and control unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram partially in block diagram form of thevehicle exhaust gas analysis system connected to a vehicle and includingan output display unit.

FIG. 2 is a schematic diagram of the gas transport system of FIG. 1.

FIG. 3 is a schematic diagram showing the details of the nondispersiveinfrared gas analyzer of FIG. 1.

FIG. 4 is a view of the chopper disk used in the infrared gas analyzerof FIG. 3.

FIG. 5 shows schematically the detector assembly of the infrared gasanalyzer of FIG. 3.

FIG. 6 is a perspective view of the infrared source in the infrared gasanalyzer of FIG. 3.

FIG. 7 is a graph showing the detector and synchronization outputsignals produced by the infrared gas analyzer of FIG. 3.

FIG. 8 is a schematic block diagram of the signal processing electronicsof FIG. 1.

FIG. 9 is a simplified schematic block diagram of the signal processingelectronics of FIG. 1 showing the gains of the signal amplifiers.

FIG. 10 is a schematic block diagram of a digital embodiment of the dataanalysis and control system of FIG. 1.

FIG. 11 is a flow chart of the program instructions for the digitalcomputation unit of FIG. 10.

FIG. 12 is a plot of the CO percentage in the exhaust gas as a functionof a computed CO ratio.

FIG. 13 is a plot of the HC content of the exhaust gas in parts permillion as a function of a computed HC ratio.

FIG. 14 is a chart showing the timing of the ambient air purge, exhaustgas sample and detector output readings as a function of vehicle enginespeed.

FIG. 15 is a schematic diagram of an analog implementation of the dataanalysis and control system of FIG. 1.

FIG. 16 is a graph showing the characteristics of the HC filter of FIG.5.

FIG. 17 is a graph showing the characteristics of the CO filter of FIG.5.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The basic vehicle exhaust gas analysis system is shown schematically inFIG. 1. A motor vehicle, shown as an automobile 10 and typicallycontaining an internal combustion engine which emits exhaust gasescontaining pollutants, has attached to its exhaust emissions system,preferably at the tail pipe, an exhaust probe 12 which is designed sothat it will not slip out of the vehicle's tail pipe under both normalvehicle vibrations and full load engine vibrations. A hand grip may beprovided as part of the probe, and the probe should be flexible enoughto extend into a curved tail pipe at least six inches if possible. Forautomobiles having dual exhausts two probes are required.

Attached to the probe 12 is a flexible gas transmission line which ispreferably in the form of a hose 14 which serves as a link between thetail pipe probe 12 and a gas transport system 16. The hose 14 ispreferably oil resistant and constructed of an internal hose materialwhich will withstand high concentrations of gases without inducing hangup of the gases within the hose. Provisions may be made for storage ofthe hose 14 when not in use. The hose should also be able to withstandthe abuse of being driven over by a heavy vehicle and return to itsoriginal shape and cross-sectional area within a short time. The hose 14should be able to withstand tail pipe gas temperatures up to 200°F andhave an inside diameter such as to permit purging of its entire lengthwithin a few seconds.

As is explained in detail in conjunction with FIG. 2, the gas transportsystem 16 contains replaceable particulate filters, a water removalsystem wherein water in the exhaust gas is eliminated through a waterdrain 18, a pumping system including a pump motor for pumping the gas tobe analyzed to a nondispersive infrared gas analyzer, and a solenoidoperated gas purge system which purges the system of exhaust gas andreplaces it with ambient air from an inlet 20. The gas transport system16 also contains a pressure switch, shown more particularly inconjunction with FIG. 2, which senses pressure to determine if ablockage has occurred in the gas transport system. A filter blockedsignal is fed to a data analysis system 24 via a signal line 22 if apressure loss in the gas transport system 16 is sensed.

The sample gas, either filtered exhaust emissions from the vehicle orambient air, is fed from the gas transport system 16 through a gas line26 into a sample cell 28, the sample cell being contained within anondispersive infrared gas analyzer 30 which is described in detail inconjujction with FIGS. 3-6. Briefly, the non-dispersive infrared gasanalyzer 30 passes a beam of infrared radiation through the sample cell28 into which the sample gas has been pumped. Two detectors, each havinga separate light filter which blocks all but the wavelengths ofinterest, respond to the absorption of infrared radiation by theparticular component of gas within the sample cell 28 which falls withinthe selected wavelengths passed by the filters, the electronic signalsfrom the detector being fed via lines 32 into a signal processingelectronics system 33, as described more particularly with respect toFIG. 8. The nondispersive infrared gas analyzer 30 also contains areference cell 34 which is fully or partially sealed and which containsa reference gas, the infrared absorption of which is compared to that inthe sample cell 28. Synchronization is provided by photosensitivedevices which provide synchronizing signals via lines 36 to the signalprocessing electronics 33. After processing, the electronic signals arefed from the signal processing electronics 33 to a data analysis system24 via lines 180a, b and 182a, b.

After the sample gas has been analyzed in the non-dispersive infraredgas analyzer 30, it is removed from the sample cell 28 through a vent38. Positioned within the vent 38 is a thermistor 40 which measures thetemperature of the exhaust gas from the sample cell 28 and provides asignal indicative of gas temperature via line 42 to the data analysissystem 24.

Since the absorption of infrared light within the sample cell isaffected not only by the temperature of the gas within the sample cell28 but by the ambient pressure, an ambient pressure sensor 44, which maybe a simple pressure transucer, produces an ambient pressure signalwhich is fed to the data analysis system 24 via a signal line 46.

A particular feature of the present invention is that the concentrationof more than one gas component may be measured simultaneously in thenondispersive infrared gas analyzer 30 by positioning a plurality ofdetectors with appropriate filters in the path of the infrared beamwhich has passed through the sample cell 28 and the reference cell 34.Typical contaminants specified by federal and state regulations at thistime are carbon monoxide (CO) and a hydrocarbon component (HC),typically hexane. Regulations also specify various limits in the amountof CO and HC in the vehicle exhaust gases at various engine speeds andunder specified load conditions. The present system is adapted tomeasure and display the amount of the selected emission contaminants inthe vehicle exhaust gas under any of a number of specified testconditions. In a typical gas emission testing installation, the systemoperator will determine from the type of vehicle under test theparticular test conditions which may be specified by local law, and thelimits for CO, HC and/or other contaminants which may be specified bylaw. In a typical installation the operator will determine theappropriate vehicle specification data from tables (including speed andload conditions and HC and CO limits) and enter the data into a vehicledata input unit 48 which provides appropriate signals as to the specificvehicle data to the data analysis system 24 via lines 50. In moresophisticated systems which may include automotive diagnostic systems,the appropriate vehicle specification data may be stored in a memoryunit and the operator merely enters the appropriate vehicleidentification code into the vehicle specification data input unit 48,the appropriate vehicle specification data being automatically suppliedto the data analysis system 24. Although not shown, the vehiclespecification data on lines 50 may also be fed directly to a displayunit 52 for display, preferably in digital form, for use by theoperator.

The display unit 52 recieves data from the data analysis system 24 vialines 53. The display unit 52 may be a hand held controller which isused by the operator during the emissions testing, the hand heldcontroller being of the type disclosed and claimed in a commonly owned,copending application of Mace Bell, Ser. No. 534,335 filed on Dec. 19,1974, and entitled VEHICLE DIAGNOSTIC HAND CONTROL. The display unit 52may, either automatically or under the control of the operator, displayany of the information which may be useful for conducting the emissionsanalysis testing. For example, the display unit may display the desiredengine speed for the emissions test. At this time the operator causesthe vehicle 10 to achieve the desired engine speed such as by depressingthe vehicle accelerator pedal. A tachometer 54 may be connected to thevehicle 10 in a known manner to produce a signal indicative of enginespeed, which is fed via a line 56 to the data analysis system 24 andwhich also may be displayed by the display unit 52. Although not shown,a dynamometer may be used to provide appropriate loading to the drivewheels of the vehicle, a signal indicative of vehicle load also beingfed to the data analysis system 24. With desired engine speed and actualengine speed being displayed to the operator by display unit 52, theoperator has a positive indication when the engine speed achieves thedesired engine test speed. Once the vehicle has achieved the desiredtest speed for a sufficient time for the sample cell 28 to contain arepresentative exhaust gas sample, the emissions analysis may beperformed and the measured CO and HC values displayed by display unit52. The display unit 52 may also display, either automatically or at therequest of the operator, the CO and HC limits so that it can easily bedetermined whether or not the vehicle meets the standards. The displayunit 52 may also contain one or more indicator lights whichautomatically indicate a system fault, such as a blockage in the gastransport system 16 or a deviation from the desired engine speed.

The data analysis system 24 may be an analog processor, but preferablyis a suitably programmed multipurpose digital computer of the type wellknown to those skilled in the art. The function of the data analysissystem 24 is to compute the CO and HC values in a manner to be describedfrom the various input data, to control the operation and timing of thegas transport system 16 by means of gas transport timing signals fed vialines 58a, b and to feed the desired data to display unit 52 via line53. Examples of both analog and digital units for performing the desiredfunctions will be described subsequently, the digital unit being shownin FIG. 10 and the analog unit in FIG. 15.

FIG. 2 shows in detail the gas transport system 16 of FIG. 1. Theexhaust gas from the vehicle under test is fed through the tail pipeprobe 12 and the hose 14 into a filtering system which consists of acoarse filter and water separator 60. The filter 60 is typically a 25micron filter. Any water in the exhaust gas is separated out and fedfrom the filter 60 through a drain line 62 which contains a pump 64, theseparated water ultimately being drained from the system via an outlet18. After coarse filtration, the exhaust gas proceeds from the filter 60to a fine filter 66 which is typically a 0.6 micron filter. Upon exitingfrom the fine filter 66, the filtered exhaust gas passes through a purgesolenoid valve 68, and then through a pump 70 where the gas passes fromthe exhaust gas transport system 16 via the line 26 into the sample cell28, which is contained within the nondispersive infrared gas analyzer30.

Connected in the line between the fine filter 66 and the purge solenoidvalve 68 is a pressure sensitive switch 74 which measures the adequacyof gas flow through the filters 60 and 66 by sensing the pressure dropacross the filters via line 72. The pressure switch is referenced toambient air and is typically set to close and produce a filter blockedsignal via line 22 when the pressure drop across the filters increasesto between 6 and 8 inches of mercury. Typically, a reduced pressure online 72 is indicative of blockage in the filters, but can also be causedby the twisting of or an obstruction in the tail pipe probe 12 or hose14. In any case, the generation of a filter blocked signal on the line22 and the display thereof by a display light in display unit 52 of FIG.1 is indicative of some malfunction in the gas transport systemrequiring action by the operator. Cleaning or replacement of the filters60 or 66 is a typical solution to an excessive pressure drop.

The purge solenoid valve 68 is a two-way valve which provides a gasinput via the pump 70 to the sample cell 28. In one position, thesolenoid valve permits passage therethrough of the exhaust gas from thevehicle under test. In the other position the purge solenoid valve 68blocks the exhaust gas line and causes ambient air from the input gasline 20 to pass through a normally open valve 72, through the purgesolenoid valve 68, and into the sample cell 28 via the pump 70. Theposition of the purge solenoid valve 68 is determined by the gastransport timing signal on the line 58a, which is provided by the dataanalysis system 24 of FIG. 1. As is described hereinafter, operation ofthe gas analysis system requires that the sample cell initially bepurged and contain ambient air, at which time a measurement is taken, byinfrared techniques, of the magnitude of the selected contaminants inthe ambient air within the sample cell. After the ambient air readinghas been taken, the purge solenoid valve 68 is actuated via a signal onthe line 58a to block the ambient air input and to permit the exhaustgas from the vehicle under test to fill the sample cell 28, at whichtime another reading of the contaminants contained in the gas in thesample cell is taken. After this latter reading, the purge solenoidvalve 68 is again actuated off the exhaust gas and to admit ambient airto purge the sample cell 28. Exhaust gas is admitted to the sample cell28 only for the time necessary to obtain the desired data and is thenimmediately purged, in order to prevent contamination of the sample cell28 by the impurities in the exhaust gas.

The pump 64, which purges water from the coarse filter 60, and the pump70, which pumps either the exhaust gas or ambient air into the samplecell 28, are both controlled by a single pump motor 74 which responds tothe gas transport timing signals on the line 58b. The pump motor 74 maybe turned off to conserve power between tests.

The flow of pump 64 should be sufficient to insure that the waterextracted from the exhaust emissions during testing of a vehicle willnot accumulate in the filter 60, but should be sufficiently less thanthe flow of pump 70 to insure that adequate exhaust gas will flow fromthe exhaust gas probe 12 into the sample cell 28. For most efficientoperation, the flow rate of the pump 70 should be such as to insure thatan inadequate sample of exhaust gas from the vehicle under test fillsthe sample cell in a few seconds. The source of ambient air 20 should belocated such as to assure that the ambient air admitted to the system isnot contaminated by exhaust emissions.

For initial calibration of the exhaust emissions analysis system, andfor calibration at selected intervals during operation, a sample gas,such as pure nitrogen or a gas containing known amounts of selectedcontaminants as provided by a container 76, is then connected to thepurge solenoid 68 through normally closed valve 78. When it is desiredto calibrate the system, valve 72 is closed and valve 78 is opened andthe calibration gas in container 76 is pumped into the sample cell 28.Calibration is performed by adjusting the signal conditioningelectronics 33 as is described hereinafter.

The heart of the exhaust emissions analysis system is the nondispersiveinfrared gas analyzer 30 and its associated components shownschematically in FIGS. 3-6. Briefly, a source of infrared radiation ofthe desired wavelength is passed alternately through the closedreference cell 34 containing a reference gas such as clean air, and thenthrough the gas sample cell 28 which contains either ambient air or theexhaust gas from the vehicle. The infrared radiation, after passingeither through the reference cell or the gas sample cell, is focusedthrough a suitable infrared filter onto a detector which produceselectrical signals which will vary as a function of the abosrption oflight in the desired wavelength band by the selected gas componentcontained in each of the cells. After conditioning by suitableelectronics as described in conjunction with FIG. 8, the output signalsfrom the detector are used to calculate the concentration of theselected gas component. By placing two or more detectors with suitablefilters in the same infrared radiation path, the concentration of two ormore gas components can be measured simultaneously with a single gasanalyzer.

Referring particularly to FIG. 3, there is shown the nondispersiveinfrared gas analyzer 30 which includes an optical bench assemblysuitably enclosed by a rigid, shock free casing 102. A source ofinfrared radiation 104 is suitably mounted in the center of the assemblyso that its output is focused into a parallel beam by a concave mirror106 mounted within the optical bench assembly. An infrared window 108having a high transmissivity in the three to five micron wavelength bandmay be positioned in front of the infrared source 104 to shield it fromstray air currents. The infrared source 104 is preferably mounted in aholding bracket and completely shielded from the optical bench andsurrounding area except for the window area. The details of the infraredsource 104 and window 108 are described in conjunction with FIG. 6.

An opaque, nonreflecting chopper disk 110 is positioned between theinfrared source 104 and the mirror 106, the chopper disk 110 beingrotated in front of the mirror 106 by a motor 112 at a suitable speed toprovide a chopping frequency to the detectors of between 32 and 55 Hz.The chopper disk 110, which is shown in greater detail in FIG. 4,contains a slot 114 through which the infrared radiation reflected bythe mirror 106 may pass, the slot 114 extending circumferentially aboutthe disk for about 90°. As a result of the rotation of the chopper disk110 in the direction shown by the arrow 157 in FIG. 4, a rotating beamof radiation is generated (the locus of the beam scribes a cylinder)which passes alternately through the gas sample reference cell 34 andthe gas sample cell 28. The cells 28 and 34 are identical tubes andinclude infrared windows 120 mounted at each end thereof to completelyseal the ends of the tubes. The infrared windows 120 are transparent tothe infrared wave band of interest, generally between 3 and 5 microns.The reference cell 34 is fully or partially sealed and contains areference gas which may be clean air, while the sample cell 28 haseither ambient air or vehicle exhaust gas fed thereto through line 26,the gas being vented from the sample cell by a line 127 through the vent38 (FIG. 1).

As the rotating infrared beam passes through the reference and samplecells, it is focused by a second concave mirror 126 onto a detectorarray 128 shown schematically in FIG. 3 and described in greater detailwith respect to FIG. 5. The detector array 128 contains two infrareddetectors 130, 132 mounted within the image of the infrared sourceformed by the mirrors 106, 126. The detectors 130, 132 are preferablylead selenide (PbSe) and are firmly attached to a mounting bracket 134which effectively shields the detectors from stray radiation. Theposition of the mounting bracket may be adjustable to assist in aligningthe optical system. Also mounted on the bracket 134 in front of eachdetector 130, 132 respectively are infrared filters 136, 138 whicheffectively shields the sensing surface of the detectors. Electricalleads 32a and 32b are connected to the detectors 130 and 132respectively to provide the detector output signals to the signalconditioning electronics 33.

For purposes of illustration, it is assumed that the filter 136 ischosen to pass a narrow band of radiation centered at 4.74 microns wherethe maximum concentration of CO occurs, so that detector 130 willgenerate alternately on signal line 32a two electrical signalsproportional respectively to the CO content in the sample cell and thereference cell, and it is assumed that the filter 138 is chosen to passa narrow band of light centered at 3.41 microns, at which wavelength themaximum concentration of the hydrocarbon hexane occurs, so that detector132 will generate alternately on signal line 32b two electrical signalsproportional respectively to the HC content in the sample cell and thereference cell. Since the infrared radiation source appears to berotating by virtue of the rotation of the disk 110, the infraredradiation passes alternately through the gas sample cell 28 and thereference cell 34, and both of the detectors 130 and 132 will beilluminated simultaneously with radiation which has passed through thegas sample 28 and then with radiation which has passed through thereference cell 34. Each of the two detectors thus produces two outputsignals separated in time, the detector signals being denoted V_(R) forthe detector output when the detector has been illuminated by theinfrared radiation which has passed through the reference cell 34, andas V_(S) for the output signal which is produced by the detector as aresult of the infrared radiation which has passed through the gas samplecell. The detector signals from each detector are shown by waveform A ofFIG. 7. Signal line 32a from detector 130 produces the V_(S) and V_(R)signals which are subsequently referred to as V_(SCO) and V_(RCO), whilesignals line 32b from detector 132 produces similar V_(S) and V_(R)signals which are referred to subsequently as V_(SHC) and V_(RHC). Thedetector signals from lines 32a and 32b are then fed to the conditioningelectronics 33 of FIG. 1 which is described in conjunction with FIG. 8.

The detectors 130 and 132 are responsive only to the radiation frominfrared source 104 which has passed through either the gas sample cell28 or the reference cell 34, and will not respond to radiation from theinfrared source 104 at other times due to a series of baffles, notshown, which prevent illumination of the detectors at times other thanwhen the infrared beam passes through the cells.

In order for the signal conditioning electronics 33 and the dataanalysis system 24 to distinguish between the sample cell and thereference cell signals V_(S) and V_(R) produced by both of the detectors130 and 132 and which appear on signal lines 32a and 32b,synchronization is provided by three light emitting diodes, each havingassociated therewith a phototransistor, the actuation of eachphototransistor by its associated light emitting diode being sychronizedto the rotation of the chopper disk 110 (FIG. 4) by a notch 144 in theouter perimeter of the chopper disk 110. Thre light emitting diodesshown in FIGS. 3 and 4 as 146, 148 and 150 are positioned on one side ofthe rotating chopper disk 110, and corresponding phototransistors 152,154 and 156 are positioned on the opposite side of the chopper disk.Corresponding light emitting diodes and phototransistors are positioneddirectly opposite each other so that when the notch 144 in the chopperdisk passes between the light emitting diode and its correspondingphototransistor, a signal is generated by the phototransistor. Thesesignals are fed to the signal processing electronics 33 on signal lines36a, 36b and 36c.

The positioning of the light emitting diodes relative to the chopperdisk 110 and the sample and reference cells 28 and 34 is shown in FIG.4. The sample light emitting diode 146 is 180° removed from the samplecell 28 so that when the slot 114 is positioned in line with the samplecell 28, the notch 144 will be positioned between the sample diode 146and its associated phototransistor 152. At this time the sample syncsignal shown at waveform B of FIG. 7 is generated by phototransistor152.

As the chopper disk 110 rotates as shown by the arrow 157 in FIG. 4, theslot 114 will pass in front of the reference cell 34, and the notch 144will pass between the reference light emitting diode 150 andphototransistor 156, which will produce the reference sync signal shownas waveform C in FIG. 7.

When the slot 114 of the chopper disk 110 is between the reference andsample cells (the position of the chopper disk shown in FIG. 4), thenotch 144 will uncover the space light emitting diode 148. At this timethe related phototransistor 154 will respond to the radiation from thelight emitting diode 148 and will produce the space sync signal shown atwaveform D in FIG. 7. The full revolution period of the chopper disk isbetween 18 and 31 milliseconds.

The sample and reference cells 28 and 34 are positioned in relationshipto the three light emitting diode phototransistor pairs so that when thelight emitting diode phototransistor pair 146 and 152 or 150-156 is atthe midpoint of notch 144, the respective sample cell 28 or referencecell 34 is at the midpoint in the slot 114. The space sync diode 148 andits phototransistor 154 are 90° removed from the sample and referencediode phototransistor pairs. The light emitting diodes are arranged sothat no signal from the diodes will significantly affect the output fromthe detectors 130 or 132.

Referring particularly to FIG. 4, the slot 144 preferably extendscircumferentially an angle X which is a maximum of 21° and has a minimumangle equal to the response time of the signal conditioning electronics33 for the rotational speed of the chopper disk. The notch 114preferably covers an angle Y which is a minimum of 89° with a maximumangle such that the signal and reference cells 28 and 34 are covered,i.e., no light is transmitted therethrough, when the space sync signalis present from light emitting diode 148 and phototransistor 154 throughnotch 144 in the position shown in FIG. 4. The angle Z shown in FIG. 4is selected so that the electrical signals from the detectors and thesignal conditioning electronics are at their peak when the notch 144produces the sample and reference sync signals by uncovering the sampleand reference light emitting diodes 146 and 150.

As further shown in FIG. 4, the radius R₂ is preferably greater thanradius R₃ by an amount sufficient to prevent illumination of the lightemitting diodes from reaching the detectors by transmission orreflection. The magnitude of the radii R₃ minus R₄ is equal to orgreater than the inside diameter of the sample and reference cells 28and 34. The sample cell and reference cell are so positioned that whenthe center point of the cell is in the center of the slot 114, theentire cell diameter will receive and pass the infrared radiation. Theradius R₁ is as large as possible to permit the slot 114 to be as largeas possible in order to stabilize the readings from the detectors priorto gating of the synchronization signals by notch 144 as is explained inconjunction with FIG. 8. The light emitting diodes and phototransistorpairs are preferably located at a common radius from the center of thechopper disk 110.

The infrared source 104 of FIG. 3 and its window 108 are shown ingreater detail in FIG. 6. The source 104 consists of a cartridge heatingelement 101 such as a Calrod unit which is inserted in a ceramic block103, the block 103 being securely mounted to the optical bench. Theceramic block may be of the type known as Alsimag which is hollowed outto form a cavity into which the heating element 101 is positioned. Theheating element 101 may be inserted into the cavity in the ceramic block103 by drilling an appropriately sized hole shown at 105 through theceramic block 103, and sliding the heating element 101 into the cavitytherethrough. The heating element 101 may be secured within the ceramicblock 103 by means of cement applied to the ends of the element 101where it meets the ceramic block 103. With the construction as describedthe only contact between the element 101 and the ceramic block 103 is atthe ends of the element 101. Since the ceramic block 103 has a lowthermal conductivity, the element 101 is relatively unaffected bytemperature changes which occur in the area surrounding the element.

The element 101 acts as a source of radiant energy when AC power isapplied thereto such as through leads 109. To assure that the element101 has a long lifetime before replacement is needed, the power appliedthereto is slightly reduced. However, because the element produces adifferent temperature and hence a different energy distribution over itsradiation spectrum as a function of applied power, sufficient power mustbe applied to assure the production of sufficient radiant energy in theband of interest, viz., between 3 and 5 microns.

The height of the opening in the ceramic block 103 through which theradiant energy from element 101 may pass is determined by the verticalheight of the opening in mounting bracket 134 of FIG. 5, through whichthe detectors 130 and 132 are exposed to the radiant energy. It ispreferred to maintain a 1:1 ratio between the height of the element 101which radiates the infrared energy and the height of the detectoropening through which the radiant energy is received.

Operation of the element 101 in the nondispersive infrared analyzer ofFIG. 3 without a front shield may result in an instability in the outputsignals from the detectors. It was discovered that the temperature ofthe element 101 and consequently its energy level may be unstable,presumably because of drafts due to the rotation of chopper disk 110only a few inches away. To solve this problem, a sapphire window 108 ispositioned in front of the window in the ceramic block, the window 108being transmissive to light in the 3-5 micron region. With the window108 installed, the signal from the detectors is very stable.

Germanium or silicon windows would not be appropriate for the window 108because of their variation with temperature. Sapphire is not effected inits transmission of radiation with temperature changes, and also has theability to physically withstand extremes in temperature.

The sapphire window 108 is mounted in a stainless steel bracket 107, thebracket being L shaped and extending along the top of the ceramic block103 where it is secured to the ceramic block by conventional hardwaresuch as a screw 107a. Alternately the bracket 107 can be secured to theoptical bench assembly to which the ceramic block 103 is also secured.The bracket 107 is relatively unaffected by heat, and maintains thesapphire window 108 in contact with the ceramic block 103 about theperimeter of the opening therein. The entire assembly 104 issubstantially immune to temperature changes and provides a very stableinfrared energy source.

Another advantage of the use of sapphire for the window 108 is thatsapphire is transmissive to visible light, thereby permitting easyalignment of the optical bench assembly. Other window materialstransmissive in the 3-5 micron region such as germanium and silicon arenot transmissive to visible light.

The signal conditioning electronics 33 for the vehicle exhaust gasanalysis system is shown schematically in FIG. 8. Two sets of signalconditioning electronics are required, one for the output signals fromeach of the detectors 130 and 132 which appear on lines 32a and 32brespectively. Only the signal conditioning electronics for detector 130is shown in detail in FIG. 8, it being understood that identical signalconditioning electronics 33' is required for the signals from detector132.

In FIG. 8, the electronic output signal from the detector 130 is fed viathe signal line 32a into a preamplifier 153, the gain of which can beadjusted by a gain adjustment input 155 which may be by means of apotentiometer or the like. The output from the detector appearing onsignal line 32a is an electronic signal of the type shown by waveform Aof FIG. 7 having peaks V_(R) and V_(S) which correspond to the timesduring which the rotating infrared beam passes through the referencecell and sample cell, respectively. The amplitude of the detectorsignals in one embodiment may be generally between 7 and 25 millivolts,peak to peak.

After preamplification in amplifier 153 and noise filtering (not shown)the detector signals pass through an automatic gain control (AGC)circuit 159, the gain of the AGC circuit 159 being adjusted as explainedhereinafter. It should be noted, however, that the gain of the AGCcircuit 159 remains constant during each rotation of the chopper disk110, that is, each combination of signals V_(S) and V_(R) during onerotation of disk 110 will have a constant gain applied thereto bycircuit 159 for reasons described hereinafter.

The V_(S) and V_(R) signals leave the AGC circuit 159 with equalamplitudes above and below ground, as illustrated by the dotted line 161in waveform A of FIG. 7. In order to reference the low edge of thesesignals to ground (as illustrated generally in waveform A), theamplified detector signals are fed to a DC restore circuit 158 where theDC level of the detector signals is referenced to ground. Another reasonfor the DC restore circuit 158 is that the infrared detectors, althoughshielded, receive continuous low level radiation from the infraredsource and from the light emitting diodes in the optical bench assembly,and consequently this continuous background light applies anindeterminant, a steady state DC component to the detector outputsignals, causing drift from ground reference. The DC restore circuit 158is synchronized with the space sync signal on the line 36b (as shown aswaveform D of FIG. 7) to provide the fixed ground reference during thetime of the space sync signal, to remove the constant DC bias componentproduced by the background light and to reference the low sideabsolutely to ground on a periodic basis.

After restoration, the detector signals are passed into a samplesynchronous demodulator circuit 160 and into a reference synchronousdemodulator circuit 162. The sample synchronous demodulator 160 issynchronized by the sample sync signal appearing on line 36a (shown inwaveform B of FIG. 7) to provide a DC signal proportional to the sensedradiation and therefore proportional to the concentration of gas in thesample cell that absorbs radiation in the band of the filter.Consequently, only the radiation which has passed through the samplecell 28 (shown at waveform A of FIG. 7 as V_(S)) is then passed throughan offset adjustment 164 and a sample output driver stage 166 to providethe output signal V_(SCO) on a line 180.

The output from the DC restore circuit 158 is also fed to the referencesynchronous demodulator 162 which is synchronized by the reference syncsignal appearing on the line 36c (shown as waveform C of FIG. 7). Due tothe synchronization, only the portion of the detector signal shown asV_(R) in waveform A of FIG. 7 is demodulated in demodulator 162. Theoutput of the reference synchronous demodulator 162 is a DC signalproportional to the concentration of CO (and other radiation absorbinggas) which is contained in the reference cell and which absorbsradiation of the band of the filter. The demodulated V_(R) signal is fedto the reference output driver 168. The output from the reference outputdriver 168 is the output signal V_(RCO) on a signal line 182.

In order to maintain the reference output signal V_(RCO) on line 182 ata fixed voltage level so that both the output signals V_(RCO) andV_(SCO) will be compatible with the digital or analog data analysissystem 24a or 24b to be described subsequently, an automatic gaincontrol feedback is applied to the reference signal V_(RCO). Thereference signal V_(RCO) is fed back via line 170 to a comparator 172 towhich a constant reference signal V_(REF) is also applied via a line174. Any difference between the reference signal V_(RCO) and theconstant reference signal V_(REF) will appear as an error signal whichpasses through feedback amplifier 176 and then via line 178 to the AGCcircuit 159. The gain of the AGC circuit 159 is changed as a result ofthe feedback circuit to maintain the reference signal V_(RCO) at aconstant value, somewhere slightly above 8 volts. It will be apparent byreference to the waveforms shown in FIG. 7 that since the gain of theAGC circuit 159 is changed if at all only when a reference signalV_(RCO) appears, and cannot be changed again until the next appearanceof a reference signal V_(RCO), the gain of the AGC circuit 159 willremain constant for the subsequent sample signal V_(SCO) which passesthrough the AGC circuit 159. In other words, once the gain of the AGCcircuit 159 is changed, it will remain constant for each pair ofreference and sample detector signals.

Duplicate signal conditioning electronics 33' for the HC detector outputsignals on line 32b provides output signal V_(SHC) on a line 180' andoutput signal V_(RHC) on a line 182'.

FIG. 9 is a simplified block diagram which is equivalent to the signalconditioning electronics 32 shown in detail in FIG. 8. In FIG. 9 thegains applied by the various amplifiers of the signal conditioningelectronics 33 to the detector output signals are shown within theblocks in order to assist with an understanding of the computationsperformed on the various signals by the data analysis system 24 of FIG.1.

Referring to FIG. 9 the output from one of the detectors is shown onsignal line 32a or 32b as signals I_(R) and I_(S) which appearalternately and are respectively the reference cell intensity equivalentdetector output and the sample tube intensity equivalent detectoroutput. The signals on lines 32a or 32b are fed to block 184 whichcontains a gain K_(P) which is equivalent to the gain of preamplifier153 of FIG. 8 as adjusted by the gain adjustment on line 155. The outputfrom block 184 is fed to the block 186 which contains a gain K_(V) whichis equivalent to the automatic gain control gain shown at 159 of FIG. 8and which is directly proportional to K_(F) × E. The output from block186 is then fed to switch 188 which represents the synchronousdemodulators 160 and 162 of FIG. 7. No gain is applied to the signals byswitch 188. The V_(S) output from switch 188 is then fed to block 190which produces a gain equal to K_(S) which is the sample signal outputamplifier gain as varied by the offset adjustment 164 of FIG. 7. A gainof unity is applied to the V_(R) output from switch 188. The V_(R)reference output voltage is then fed back to comparator 192 where it iscompared to the V_(REF) reference voltage, the comparator generating anerror signal E which is equivalent to the difference between V_(R) andV_(REF). The error signal E is fed to block 194 where the gain K_(F) ofthe feedback amplifier (176, FIG. 7) is applied thereto, and the outputfrom block 194 is fed to block 186 to vary the gain K_(V) therein inaccordance with the output from block 194, and which is proportional toK_(V) × E.

If the gas sample cell 28 is filled initially with ambient air, and theinfrared radiation beam is rotated and passed through both the samplecell 28 and the reference cell 34 while ambient air is contained in thegas sample cell, the reference output voltage V_(R) and the sampleoutput voltage V_(S) at that time can be represented as V_(R) AIR andV_(S) AIR. If the purge solenoid 68 of FIG. 2 is then actuated to blockthe ambient air input and admit exhaust emissions from the vehicle undertest into the sample cell 28, and if the rotating infrared radiationbeam is then passed through the reference and sample cells while thesample cell is filled with exhaust gas, the reference output voltage andsample output voltage at that time can be represented as V_(R) GAS andV_(S) GAS. It will be shown that by using the data analysis system 24(FIG. 1) to perform the calculation: ##EQU1## there is produced a ratiowhich is equivalent to the amount of a particular contaminant, CO or HC,in the emission gas with respect to the amount of the particularcontaminant in the surrounding atmosphere and contained in the ambientair initially admitted into sample tube 28. If, instead of ambient air,the sample tube is initially filled with a reference gas which containszero concentration of the particular contaminant gas, the ratio shown byEquation 1 will be an absolute ratio of the amount of the particularcontaminant to a sample which contains none of the contaminant.

By performing the computation shown in Equation 1 such as by a digitalcomputer programmed in accordance with techniques known to those skilledin the art, or by performing the calculation in an analog manner, theamount of the particular contaminant is uniquely measured. The ratiocalculated by Equation 1 is then compensated for nonlinearities in thegas sampling and measurement system, and is then further corrected forchanges in ambient pressure and gas temperature. The compensated ratiois then fed to the display unit 52 of FIG. 1 and is equivalent to thepercentage of CO in the exhaust gas or the parts per million of HC inthe exhaust gas.

FIG. 10 shows a preferred embodiment of the emissions analysis system inwhich the computations and corrections are performed by a digital dataanalysis system 24a which includes a digital data computation unit 208.The digital computation unit 208 may be a general purpose digitalcomputer. A flow chart illustrating representative program stepsperformed by the digital data computation unit 208 is described in FIG.11.

FIG. 10 contains a signal multiplexing unit 206. Feeding into the signalmultiplexing unit 206 are the output signals V_(SCO), V_(RCO), V_(SHC)and V_(RHC) from the signal conditioning electronics 133 and 133' ofFIG. 8 on lines 180, 182, 180' and 182', respectively. Also fed into thesignal multiplexing unit 206 are the filter blocked signal on the signalline 22, the gas temperature signal on the signal line 42, and theambient pressure signal on the signal line 46.

The signal multiplexing unit 206 receives each of the analog inputsignals and feeds the selected signal at the proper time to datacomputation unit 208 through an analog-to-digital converter 210 underthe control of the address control signals appearing on line 212.

Also fed directly to the data computation unit 208 are the vehiclespecification data signals on lines 50a, b and the engine speed signalon signal line 56. The engine speed signal may be generated as shown inFIG. 1 by means of a tachometer, or a counter 213 may be used as shownin FIG. 10 to generate a signal on line 56 which is proportional toengine speed. Fed to the counter 213 are clock pulses from a source, notshown within data computation unit 208, and a series of timing pulsesfrom the low coil of the vehicle under test on a line 215. The counter213 is adapted to be actuated by a selected low coil pulse, and stoppedby the next low coil pulse, the number of clock pulses counted thereinbetween coil pulses being proportional to engine speed. For example, ifthe vehicle under test has an 8 cylinder engine, the time between twolow coil phases is equal to one-fourth revolution of the engine, or 90°.The data computation unit 208 receives the count from counter 213 online 56 and computes the engine speed therefrom, the computation being afunction of the number of engine cylinders, which data has been fed tothe data computation unit via lines 50a, b with the vehiclespecification data.

The data computation unit 208 as previously indicated may be a generalpurpose digital computer. The program instructions and necessaryadditional data such as constants are stored in a read only memory 214which controls the operation of the data computation unit 208. Temporarystorage during computation is provided by a random access memory 216which is in communication with the data computation unit 208. Theoutputs from the data computation unit 208 include an input to thedisplay unit 52 via output line 53e and the generation of gas transporttiming signals on signal lines 58a, b, the latter signals being fed tothe exhaust gas transport system 16 described in detail in FIG. 2 andwhich signals control the actuation of the purge solenoid valve 68 andthe actuation of pump motor 74 to cause either ambient air or exhaustgas to fill the sample tube 28 at the proper times.

The data computation unit 208 of FIG. 10 accepts the signals from thesignal multiplexing unit 206, performs the computation shown by Equation1 in a manner such as is shown in the flow chart in FIG. 11, andcorrects the calculated ratio for nonlinearities, and for ambientpressure and gas temperature.

Equation 1 is derived as follows. According to the Beer-Lambert Law:

    Equation 2: I.sub.υ =  I.sub.υ .sbsb.o.spsb.e .sup.-.sup.σ(υ).sup.cl

where J.sub.υ = intensity of light at frequency υ after transmissionthrough the gas

I.sub.υ.sbsb.o = initial light intensity

σ.sub.υ = absorption coefficient of the gas at frequency υ

C = concentration of the gas by volume

l = path length through the absorbing gas.

σ.sub.υ is a function of pressure and temperature as well as frequency.

While the filters 136 and 138 which shield the detectors 130 and 132have a finite band spread and do not absorb completely at onewavelength, Equation 2 is sufficiently accurate when a high qualityfilter is used to determine a mean absorption coefficient σ.sub.υ. Sinceair is composed almost entirely of diatomic gases, oxygen and nitrogen,which do not absorb infrared radiation, if a radiation beam of fixedintensity is measured at the 4.74 micron (CO) and 3.41 (HC) micron bandsafter being passed through a sample tube containing first ambient airand then a vehicle emission gas, the concentration of CO and HC in thegas can be computed from the change in signals. The c and l terms inEquation 2 may be determined by calibration with the sample tube filledwith a calibration gas having a known concentration of the gases.

For purposes of deriving Equation 1 and showing its relationship toEquation 2 it can be seen from FIG. 9 that

    Equation 3: V.sub.R = I.sub.R × K.sub.P × K.sub.V

and

    V.sub.S = S.sub.R × K.sub.P × K.sub.V × K.sub.S.

since one of the unique features of the present invention is theavoidance of the need to manually correct for changes in span (range ofgross input signal magnitude) and zero settings, four measurements aremade. The reference and sample voltages V_(R) and V_(S) are made withambient air in the sample tube, and at a later time the reference andsample voltages V_(R) and V_(S) are made with the exhaust gas in thesample tube. Consequently Equations 1 and 3 can be combined intoEquation 4 as shown below: ##EQU2##

K_(P) and K_(V) are independent of whether emission gas or ambient airare in the sample tube and will change only with time. Since the sampleand reference measurements are made almost simultaneously, terms may becancelled out as shown below in Equation 5: ##EQU3##

If the gain K_(S) shown in block 190 of FIG. 9 is a simple electroniccircuit with near zero drift, and if the measurements with ambient airand exhaust gas in the sample tube are taken relatively close togetherin time, K_(S) may also be cancelled out, leaving Equation 6: ##EQU4##

The Beer-Lambert Law of Equation 2 may now be used to rewrite Equation 6in terms of initial source intensity as shown below in Equation 7:##EQU5##

Since a reference gas such as clean air is always contained in thereference cell 34, Equation 7 converts into Equation 8 as shown below:##EQU6## which simplifies to ##EQU7##

On the left-hand side of Equation 9 are only measured parameters and onthe right-hand side are only the constants e, σ and - and the desiredquantities C_(GAS) - C_(AIR)). No variables requiring zero or spanadjustments remain.

The Equation 9 can be plotted as Equation 10 shown below: ##EQU8##

Equation 10 is plotted from empirical data using known gasconcentrations for CO in FIG. 12 and for HC in FIG. 13, which plotsprovide the basic calibration curve of the emissions analyzer. Thecurves are for a temperature of 30.0°C and a pressure of 29.75 inches ofmercury. All individual systems produced in accordance with theinvention are set to the same curve which is stored in the processormemory, block 214 of FIG. 10. In other words, by computing Equation 10to produce a ratio signal indicative of the CO or HC ratio as showninitially in Equation 1, and by calibrating the computed ratio inaccordance with the appropriate curve shown in FIG. 12 (CO) or FIG. 13(HC), the calibrated ratio signal for CO is provided in percentconcentration CO and the calibrated ratio signal for HC is provided inparts per million HC.

Since the value of σ is actually affected by ambient pressure and gastemperature, these two parameters are measured and compensation isprovided, using the computer program stored in random access memory 214of FIG. 10. The pressure and temperature corrections for CO and HC areshown in Equation 11:

Equation 11:

    CO = CO calibrated ratio (uncorrected) × [1.8256 + 0.0058 T.sub.G - 0.0336 P.sub.A ]

    hc = hc calibrated ratio (uncorrected) × [1.9336 + 0.0022 T.sub.G - 0.0336 P.sub.A ]

where T_(G) = gas temperature (°C) of exhaust gas from thermistor 40 ofFIG. 1, and P_(A) = ambient pressure (mm Hg) from sensor 44 of FIG. 1.

Once the computed ratio has been calibrated according to the curves ofFIGS. 12 or 13, and has been corrected for pressure and temperature inaccordance with Equation 11, the resultant computations of CO and HCvalues from data computation unit 208 of FIG. 9 are sent to the displayunit 52 where the percentage of CO and/or the amount of HC in parts permillion is displayed in digital or numerical format for the operator. Ifdesired, the CO and HC values can be compared with the limits for CO andHC contained as part of the vehicle specification data and a displaylight illuminated to indicate if the vehicle under test is out ofspecification for amounts of either CO, HC or both.

It is important that the measurements of V₄ and V_(S) using ambient airin the sample cell and the measurements of V_(R) and V_(S) using exhaustgas in the sample cell be made reasonably close together in time inorder to prevent changes in the radiation from the infrared source fromchanging the output from the detector.

FIG. 11 is a simplified flow chart showing representative program stepswhich may be contained in the program instructions stored in randomaccess memory 214 of FIG. 10 and which control the computations in datacomputation unit 208 and the transmission of the input signals from thesignal multiplexing unit 206 to the analog-to-digital converter 210 andthen to the data computation unit 208. The outputs from the datacomputation unit 208 to the display unit 52 via line 53e and theinitiation of the gas transport timing signals on lines 58a, b are alsocontrolled by the program steps shown in FIG. 11. It will be apparent toone skilled in the art of digital computer programming that variousother program steps and implementations of the invention may beperformed depending on the specific construction and operation of thedata computation unit 208.

Although not shown in the flow chart of FIG. 11, the engine speed may becontinuously monitored, i.e., compared with the desired vehicle enginespeed illustrated as the engine speed reference signal in conjunctionwith the vehicle specification data on lines 50a, b of FIG. 1. If theactual engine speed is out of the desired range, i.e., a range centeredabout the desired engine speed, the HC and CO values are not displayedin display unit 52 and an indicator light in display unit 52 may beturned on, indicating that the engine speed must be adjusted. The filterblocked signal on the line 22 may also be continuously monitored, and ifpresent the computations terminated and the system purged with ambientair. The manner of implementation of these program steps may be inaccordance with programming techniques which are well known to thoseskilled in the art and are therefore not described in detail.

If desired the V_(R) GAS and V_(S) GAS signals may be continuouslymonitored during the time that these readings are made, new V_(R) GASand V_(S) GAS signals being taken approximately twice per second. Thecomputations of the HC and CO values may also be continuously updated inlike manner, and averaged on a continuous basis so that the valuesdisplayed in display unit 52 are the average values displayed in displayunit 52 are the average values of HC aND CO. Other changes in theprogram steps will be apparent to those skilled in the art.

FIG. 14 shows a typical timing chart for the emissions analysis of theexhaust gas from an automobile as performed by the system of FIG. 1 andthe data analysis system 24a of FIG. 10. Once the vehicle is in properposition for the test, the operator actuates the gas analysis system andgas transport timing signals are fed from the data analysis system 24avia lines 58a, b to ensure that the sample cell is purged with ambientair. If the system has not been left purged with ambient air in animmediately preceding cycle, sufficient time is allowed so that it isassured that the sample cell is first filled with ambient air. At thesame time the operator installs the probe 12 on the vehicle tail pipeand enters the vehicle specification data into the data analysis system24a via lines 50a, b, Display unit 52 displays the desired engine speedas digital data which is read by the operator, and the operator thencauses the vehicle to accelerate to the desired engine speed, in thisexample high cruise. The actual engine speed is fed to the data analysissystem via line 56. Once the engine reaches the desired speed, and withthe sample cell filled with ambient air, the V_(R) AIR and V_(S) AIRreadings are taken and sent to the data analysis system for storage.

Once the V_(R) AIR and V_(S) AIR readings have been made, gas transporttiming signals are fed from data analysis system 24a to the purgesolenoid valve 68 of FIG. 2 to cause the solenoid valve to block theambient air input to the sample cell and pump exhaust gas from probe 12and hose 14 into the sample cell. After a time sufficient to assure thatthe sample cell is filled with exhaust gas, the V_(R) GAS and V_(S) GASreadings are taken.

It should be noted that during acceleration, raw fuel is fed into theengine, and any HC or CO content measurements made on the exhaust gasfrom the vehicle at this time will be very high and unreliable. Theoperator must wait until the excess fuel is burned and the emissions gasstabilizes before valid readings of CO and HC can be made. A time lapseof seven seconds is considered adequate after acceleration for reliablemeasurements to be made.

Once the desired readings are taken, the sample cell is again purgedwith ambient air to remove the exhaust gas, but no additional V_(R) AIRor V_(S) AIR readings are required. During the purge time the operatorcauses the vehicle to decelerate to a low cruise condition, the desiredengine speed having been displayed in display unit 52 after the initialV_(R) GAS and V_(S) GAS readings were taken. After the ambient airpurge, exhaust gases are again admitted to the sample cell and new V_(R)GAS and V_(S) GAS readings taken for the low cruise condition. The cycleof ambient air purge and exhaust gas sample is repeated for engine idlespeed, and additional V_(S) GAS and V_(R) GAS readings taken for theidle condition. The system is then finally purged with ambient air andset in a standby mode, e.g., pump motor 74 of FIG. 2 may be turned off,until the next vehicle is in position and the cycle is repeated.

The ratio calculations may be made in the data computation unit 208 anddisplayed either during each purge-sample cycle, or when the cycle hasbeen terminated. For the example given, three HC and CO ratiocalculations will be made, one for each engine speed. Obviously thenumber of speeds at which measurements of emission contaminants are madecan be varied.

It is important that the exhaust gas sample is fed into the sample cellfor only the time required to obtain readings, the sample cell beingpurged with ambient air at all other times to prevent contamination ofthe cell by pollutants in the exhaust gas.

An analog implementation of the system of this invention is shown inFIG. 15. The various input signals in FIG. 1 are fed to an analog dataanalysis system 24b and which is equivalent to the digital data analysissystem 24a of FIG. 10. Referring to the FIG. 15, the V_(SCO) and V_(RCO)signals appearing on signal lines 180 and 182 are fed respectivelythrough normally closed switches 294 and 296, whose operation will bedescribed subsequently, and then through switches 300 and 302respectively, the V_(S) signal being fed to lines 304 or 306 labeledV_(S) AIR and V_(S) GAS respectively depending on the position of switch300, and the V_(R) signal being fed to lines 308 or 310 which arelabeled V_(R) AIR and V_(R) GAS respectively, depending on the positionof switch 302. The position of switches 300 and 302 is determined by theposition of switch 312 which is actuated manually by the operator of thesystem. Switch 312 has two positions, PURGE and SAMPLE, and is biased sothat unless it is held by the operator in the SAMPLE position, it willreturn to the PURGE position. Switch 312 may be a time delay switchwhich returns to the PURGE position after a selected time such as eightseconds after being moved to the SAMPLE position. While in the PURGEposition, a signal is fed via line 58a' to purge solenoid 68 to causethe purge solenoid to admit ambient air into the sample cell. Whileswitch 312 is in the PURGE position, the switches 300 and 302 arenormally biased as shown in FIG. 15, i.e., in contact with lines 304 and308 respectively.

Assuming that switch 312 is in the PURGE position, the V_(S) AIR signalon line 304 is fed to a sample and hold circuit 314 where the V_(S) AIRsignal is stored. Likewise the V_(R) AIR signal on line 308 is fed to asample and hold circuit 316 where it is stored.

When a vehicle is ready to be tested, and has reached the desired testspeed as shown by an indication on the display unit 52, the operatormoves switch 312 to the SAMPLE position. At this time a signal is fedvia line 58a" to the purge solenoid valve 68 to cause the ambient airinlet to be blocked and exhaust gas to be admitted to the sample cell28. A timer 318 is also actuated, and after a suitable time such as 7seconds to assure that the sample tube is filled with exhaust gas, thetimer 318 times out causing relays 320 and 322 to be actuated. Actuationof relay 320 closes normally open switches 324 and 326 whose operationwill be described subsequently, and actuation of relay 322 movesswitches 300 and 302 to feed the V_(SCO) and V_(RCO) signals on lines180 and 182 to lines 306 and 310 respectively.

Connected to the output from sample and hold circuit 314 and alsoreceiving the signal on line 308 is a multiplier 328 which generates theproduct N₁ V_(S) AIR × V_(R) GAS. Connected to the output from sampleand hold circuit 316 and also receiving the signal on line 306 is amultiplier 330 which generates the product N₂ = V_(S) GAS × V_(R) AIR.The N₁ and N₂ outputs from multipliers 328 and 330 are fed to a divider332 where the division N₂ /N₁ is effected. This division results in theRATIO of Equation 1. The RATIO signal from divider 332 is fed to anonlinear function generator 334 where the compensation to the RATIOsignal in accordance with the curve of FIG. 12 is performed. Functiongenerator 334 may be a simple diode network. The output from thenonlinear function generator 334 is then fed to pressure and temperaturecompensator 336 where the computed value of CO is compensated forpressure and temperature. To accomplish this compensation, the gastemperature signal on line 42 and the ambient pressure signal on line 46are fed respectively through scaling amplifiers 338 and 340 intocompensator 336, which provide the constants for T_(G) and P_(A) inEquation 11, which are then summed in a summing junction (in compensator336) with a fixed signal representing the constant (1.8256) in Equation11. The output of the summing junction is multiplied with the output ofthe function generator 334 in an analog multiplier (in the compensator336).

The output from compensator 336 is the resultant CO measurement signalwhich is then fed through switch 324, now closed because of actuation ofrelay 320, and via line 53a to display unit 52. Once the CO measurementis made and displayed on display unit 52, the operator moves switch 312back to PURGE, or switch 312 will move back to PURGE after a selectedtime delay, thereby opening switch 324 so that no signal can thereafterpass therethrough.

The HC measurements are made by analog apparatus 342 which is identicalto that just described (except for the nonlinear function, which is thatof FIG. 13) and which receives inputs V_(SHC) via line 180' and V_(RHC)via line 182', and also receives temperature and pressure input signalsvia scaling amplifiers 338' and 340'. The computed HC signal is fed viaswitch 326 and line 53b to display unit 52 at the same time that the COsignal is fed to the display unit.

If, during the measurements, the engine speed on line 56 deviates fromthe engine speed reference signal on line 50a by an amount determinedwithin a comparator 344, or if a filter blocked signal appears on line22, OR gate 346 is actuated and relay 348 is energized to open switches294 and 296 so that a zero output signal is produced on lines 53a and53b, and switch 298 is closed to cause a system fault signal to bepassed to display unit 52 via line 53c.

Vehicle specification data in this embodiment which appears on line 50bmay be fed directly to display unit 52 via line 53d.

FIGS. 16 and 17 show respectively the filter characteristics of thefilters 138 and 136 of FIG. 5 which may be used to pass therethrough thespecified wavelengths for measurement of HC and CO. The particularwavelengths chosen and described herein were selected by a governmentalagency for emissions analysis testing. Since the components inautomobile exhaust emissions of hexane and carbon monoxide occur atother wavelengths than those described, it is obvious that otherwavelengths may be chosen to test for HC and CO components in theexhaust gas.

It will also be apparent to those skilled in the art that more than twodetectors may be used in the system and that tests may be made for othercontaminants such as carbon dioxide, acetylene, methane or nitrous oxide(NO), by simply replacing the filters with other filters which passradiation at the desired wavelengths.

While the invention has been described with respect to preferredembodiments thereof, it will be apparent to those skilled in the artthat changes and modifications may be made to the construction andarrangement of parts and the operation thereof without departing fromthe scope of the invention as hereinafter claimed:

We claim:
 1. A gas analysis system for measuring the amount of aselected contaminant in a sample gas comprising:a sample cell adapted tocontain a gas; means for generating a beam of infrared radiation; asolenoid valve having first and second positions; means for passing airthrough said solenoid valve to said sample cell when said valve is insaid first position and passing said sample gas through said solenoidvalve to said sample cell when said valve is in said second position;means including a data control system for moving said valve between saidfirst and second positions, to first purge said sample cell with air andto thereafter remove the air and fill said sample cell with the samplegas, and for passing said beam through said sample cell during the timethat said cell contains air and producing a first electrical signalindicative of the absorption of said radiation by the selectedcontaminant in said air, and for passing said beam through said samplecell during the time that said cell contains said sample gas andproducing a second electrical signal indicative of the absorption ofsaid radiation by the selected contaminant in said sample gas, said datacontrol system being responsive to said first and second electricalsignals for producing an output signal indicative of the amount of saidselected contaminant in said sample gas and purging said sample cellwith air immediately after generation of said output signal by movingsaid valve from said first position to said second position in responseto generation of said output signal.